The long-term goals of the project are to (1) Characterize HIV-related high-risk behaviors (heterosexual, drug abuse, homosexual) among commercial sex male clients (CSMCs) population;determine the prevalence and interacting effect of these behaviors, and identify associated factors from both community and individual level. (2) Determine, through the community-based research, HIV prevalence and incidence among CSMCs. Identify the risk and mechanisms of spreading HIV/STD through this "bridge" population to general public;and explore effective community-based intervention strategies (3) Investigate their high-risk behavior change after HIV diagnosis and the risk of secondary HIV infection;and address issues associated with a nti retro viral therapy. The specific aims of this study are to (1) Qualitatively study HIV-related high-risk behavior, including heterosexual, homosexual and drug abuse, as well as their interaction on unprotected heterosexual practice among commercial sex male clients (CSMCs) with the expanded Health Belief Model (HBM). (2) Quantitatively determine the characteristics of CSMCs;the prevalence of HIV-related risky behaviors;and factors contributing to high-risk heterosexual behavior in the aspects of community environment, social network and individual characteristics/knowledge/perception/other risk-taking behavior among CSMCs;and determine the prevalence of HIV infection in CSMCs and their associated factors. Initially, two cities with HIV epidemic, one due to commercial sex and another due to both commercial sex and drug abuse, will be selected and mixed method will be used. In the first phase, qualitative study will be conducted among CSMCs at both cities to explore their HIV-related high-risk behavior, refine conceptual framework and develop instrument for subsequent quantitative study. Then community-based cross-sectional study will be conducted among CSMCs for the prevalence of HIV high-risk behavior, prevalence of HIV infection and associated factors from community and individual perspectives. Our study will be one of pioneer studies in China that address male heterosexual HIV transmission, also a necessary step for exploring effective community-base intervention. Given the catastrophic consequence of HIV epidemic, and valuable information that will be collected from this study which can be applied to other developing countries facing similar epidemic pattern, the public health significance of this study perfectly fit the mission of NIH.